Iridium tip electrode and method of making the same



J. K. LEVER April 18, 1967 IRI'DIUM TIP ELECTRODE AND METHOD OF MAKING THE SAME Filed Oct. 20, 1964 INVENTOR! JUHN K. LEVER BYM JITTYS.

United States Patent Ofiiice 3,315,113 Patented Apr. 18, 1967 3,315,113 IRIDIUM TIP ELECTRODE AND METHOD OF MAKING THE SAME John K. Lever, Perrysburg, Ohio, assignor to Champion Spark Plug Company, Toledo, Ohio, a corporation of Delaware Filed Oct. 20, 1964, Ser. No. 405,154 4 Claims. (Cl. 313-136) This invention relates to a tip electrode and the method of making the same, and more particularly, to a tip electrode which can be incorporated into a spark plug assembly.

It is desirable to construct the center electrodes of spark plugs used in highly corrosive conditions of a material having a high degree of corrosive resistance. Spark plugs made of expensive corrosion resistant materials generally employ small sized electrodes and are often referred to as fine wire spark plugs. The center electrode of a typical fine wire spark plug terminates at its firing end in a tip constructed of a highly corrosive-resistant material. Platinum and its alloys have been used for this purpose in the prior art. Such materials can be cold worked, and in prior art electrodes having a platinum or alloy tip, a head or shoulder portion is formed on a platinum rod by cold up-setting.

It has been discovered that other elements, for examples, iridium, tungsten, molybdenum, ruthenium, and rhodium are often superior to platinum for use in a spark plug electrode subjected to arcing in the corrosive atmosphere of an internal combustion engine. However, these elements are much more diflicult to work than is platinum because of their fibrous characteristics which render them very brittle. It is virtually impossible to form shoulders or heads, for example, on an iridium rod by a cold upsetting operationv The remaining portion of the center electrode immediately above the tip electrode often consists of a conductor such as silver. When platinum tips are used, a chemical surface bond is formed between the platinum tip and the silver. However, this is not true when, for example, an iridium tip is used. Because no appreciable chemical or metallurgical bonding occurs, the iridium tip has a tendency to move or rotate in its seat.

Attempts have been made to place a spherical head on an iridium rod, which head is then embedded in the silver portion of the center electrode. However, when the iridium rod is heated to the melting point and then molded into the desired shape, recrystallization occurs. Recrystallized fibrous metals are extremely brittle and as such, are very susceptible to tensile or vibrational failures.

Because tips for electrodes are very small, usually in the order of inch in length, it is extremely difiicult to form a spherical head or other projection on the end of, for example, an iridium rod by a melting and molding procedure whereby the head is concentric with the shank portion of the rod.

It is an object of the present invention to provide a new and improved method for producing a tip electrode having an outwardly extending section.

Another object of this invention is to provide a tip electrode having an outwardly extending section, of a predetermined configuration, for use in a spark plug assembly.

Still another object of the present invention is to provide a spark plug electrode having an iridium tip positioned at the firing end of the electrode.

Still another object of the present invention'is to provide a novel tip electrode having a configuration whereby a mechanical bond is obtained between the tip and an adjacent part of a spark plug center electrode.

A further object of the present invention is to provide a tip electrode having an outwardly extending shoulder section by a novel method whereby the tip is not made brittle by recrystallization.

Further objects and advantages will become apparent from the following specification and drawings in which:

FIG. 1 is a side view in elevation of a spark plug assembly with a portion of such assembly broken away and shown in section;

FIG. 2 is a side view in perspective showing a tip electrode according to the instant invention;

FIG. 3 is a diagrammatic view, with parts broken away, showing an apparatus used in practicing the method of the present invention for producing a tip electrode;

FIG. 4 is a side view in elevation of a second embodiment of a tip electrode according to the instant invention; and

FIG. 5 is a side view in elevation, with a part broken away and shown in section, illustrating still another embodiment of a tip electrode constructed according to the instant invention.

As previously stated, the present invention relates to a novel tip electrode and a method for making the same. The tip comprises a longitudinal extending firing end, a head end longitudinally spaced from and axially aligned with the firing end, and an outwardly extending, intermediate shoulder section, positioned between and coaxially aligned with the firing end and the head end. The intermediate section is designed with a predetermined configuration to achieve a high degree of stability when used in a spark plug assembly.

The method of shaping a tip electrode having an outwardly extending shoulder section from a cylinder having opposed ends, comprises the steps of applying lateral support to upper and lower cylindrical surfaces of a rod constructed from an element selected from the group consisting of iridium, tungsten, molybdenum, ruthenium, and rhodium, heating an intermediate portion of the rod to a predetermined working temperature, and applying a compressive force to opposed ends of the rod. The opposed ends of the rod are moved toward each other a predetermined distance thereby forming the outwardly extending shoulder section.

Referring to FIG. 1, a spark plug assembly is generally indicated by the reference numeral 10. The spark plug assembly 10 is of the fine wire variety and is used where highly corrosive conditions are present. The spark plug assembly 10 comprises an annular insulator 11 hav ing an axial stepped bore 12 extending therethrough. A center electrode assembly 13 is fixed in the stepped bore 12 and a metallic sleeve 14 surrounds the insulator 11. An outer metallic shell 15 surrounds the insulator 11 and one one or more ground electrodes 16 extend inwardly at the lowermost end 17 of the metallic shell 15. The ground electrode 16 is in electrical communication with the metallic shell 15.

The lower portion of the center electrode assembly 13 consists of a sealing material 18 which is in contact with the uppersurface of an annular metallic sheath 19. The

sheath 19 is constructed of, for example, a nickel alloy and has a head portion 20 which seats on a shoulder 21 within the bore 12 of the insulator 11. The head portion 20 of the metallic sheath 19 has an opening 22 which communicates with a central opening 23 in the metallic sheath 19. An electrode 24 projects upwardly out of the opening 22 into the sealing material 18 with the lower part of the electrode 24 being in electrical communication with a core 25 which fills the central opening 23 of the metal sheath 19. In a spark plug assembly 10, the core 25 may consist of silver, copper, or other materials of high thermal conductivity.

The metallic sheath 19 has a radially inwardly extending annular flange 26 on its lower end. The flange 26 surrounds opening 27 therethrough and has an upper surface 28 of a predetermined configuration.

A tip electrode 29, according to the present invention, is coaxially aligned with the center electrode 13 and extends through the opening 27.

Referring to FIG. 2, the tip electrode 29 comprises a cylindrically shaped and longitudinally extending firing end 30, a cylindrically shaped head end 31 longitudinally spaced from and axially aligned with the firing end 30 and an enlarged intermediate shoulder section 32. Normally, the length of the firing end 3! is greater than the length of the head end 31. It is important that the enlarged shoulder section 32 be concentric with the longitudinal axis of the remainder of the tip electrode 29. If concentricity is not obtained, the tip electrode 29 cannot be used in the spark plug assembly 10 and must be scrapped. The tip 29 is constructed of, for example, iridium which is an extremely expensive metal and commercial economics demand that such waste be kept at a minimum.

The enlarged shoulder section 32 of the tip electrode 29 has a lower surface 33 constructed, in the preferred embodiment, of a shape which is complementary with the predetermined configuration of the upper surface 28 of the flange 26 on the metallic sheath 19. Referring to FIG. 1, the head end 31 and the enlarged shoulder section 32 are positioned in the central opening 23 of the sheath 19 and the core 25 is cast around the head end 31 and the enlarged shoulder section 32. The surface 33 of the shoulder section 32 is seated on the upper surface 28 of the flange 26. The tip electrode 29 is coaxially aligned with the opening 27 and the firing end 30 extends through such opening.

It has been found that only a relatively weak bond can be formed between, for example, an iridium surface and a silver surface. It has further been found that a spherical surface on an iridium tip is very difficult to bond to a silver core. The mating surface of the silver core of such an assembly acts as a socket and when the assembly is jarred, as during normal usage, the spherical portion easily breaks loose and swivels in the socket and thus changes the spark gap. This is, of course, undesirable because it changes the operating characteristics of the spark plug.

According to the instant invention, the head end 31 of the tip electrode 29 extends upwardly into the cast core 25 and thereby prevents swiveling of the enlarged shoulder section 32.

FIGS. 4 and illustrate two different embodiments of tips constructed according to the instant invention. While the tip electrode 29, illustrated in FIG. 2, has a shoulder section 32 of elliptical vertical cross section, the instant invention is not limited to shoulder sections of this one configuration. The configuration of the shoulder section may vary depending on the use to which the tip electrode 29 is to be placed. For example, FIG. 4 illustrates a tip electrode 29:! constructed according to the instant invention which comprises a firing end 30a, a head end 31a, both of which are similar to the related elements disclosed in FIG. 2, and an enlarged shoulder section 32a of diamond shaped vertical cross section. The lower surface 33a of the shoulder section 32a tapers upwardly and outwardly away from the upper end of the firing end a. When the tip electrode 29a is used in a spark plug assembly wherein the flange 26 of the metallic sheath 19 has a complementary upper surface (not shown), a wedging action is created and the swiveling action above referred to is minimized.

FIG. 5 illustrates another embodiment of the instant invention wherein a tip electrode 2% has a firing end 39b, a head end 31d, both of which are similar to the corresponding elements in the embodiments illustrated in FIGS. 2 and 4, and an enlarged shoulder section 32b of an inverted cup-shaped configuration. The upper surface 35 of the enlarged shoulder section 3212 has a generally spherical configuration. The lower surface 33b extends downwardly and outwardly from the upper end of the firing end 36b in an inverted cup configuration to form a recess 36. When used in a spark plug assembly, an upper surface (not shown) of the flange 26 of the metallic sheath 19 fits into the recess 36 to form a tight mechanical connection.

In the embodiments described, the firing end 30 of the tip electrode 29 defines a spark gap with one or more ground electrodes 16. Because the elements, iridium, tungsten, molybdenum, ruthenium, and rhodium are quite hard, they are diflicult to forge or fabricate and can be worked hot only with extreme care and control. If, for example, iridium is heated to a point which exceeds its recrystallization temperature, which is approximately 1800 F., the metal will recrystallize and become extremely brittle. Furthermore, because of the economics of the situation, any method used to fabricate a tip must keep waste to a minimum.

Referring to FIG. 3, there is shown a preferred form of apparatus for performing the method of the instant invention. The apparatus comprises a press generally indicated by the reference numeral 37. The press 37 has a movable platen 38 and a fixed bed 39. The movable platen 38 may be moved toward and away from the fixed bed 39 by any suitable means such as by a connecting rod 40 which is connected to the upper portion of the movable platen 38.

The connecting rod 40 can be actuated by, for example, a hydraulic cyclinder (not shown) as is common in the art. An upper die 41 is releasably secured to the movable platen 38 and a lower die 42 is likewise secured to the fixed bed 39.

The upper die 41 has a cylindrical recess 43 which is coaxially aligned with a similar recess 44 in the lower die 42. The recess 43 has a bottom 45 and cylindrical sides 45. Likewise, the cylindrical recess 44 has a bottom 47 and cylindrical sides 48. A step-down transformer 49 supplies current to the upper and lower dies 41 and 42 by lead-in wires 50 and 51.

The tip electrode 29, shown in FIG. 2, is produced by placing opposite ends 53 and 54 of a rod 52 of predetermined length into the recesses 43 and 44 of the die blocks 41 and 42. As shown in FIG. 3, the recess 44 is deeper than the recess 43. This difference in depth of respective recesses 43 and 44 determines the position of the enlarge-d shoulder section 32 (shown in dashed lines in FIG. 3) which will later be formed. The cylindrical sides 46 and 48 of the recesses 43 and 44 laterally support the upper and lower cylindrical surfaces of the rod 52 during downward movement of the movable platen 38 thereby preventing expansion of the upper and lower portions of the rod 52.

An electrical circuit is established between the upper die 41 and the lower die 42 through the rod or cylinder 52. The cross-sectional area of the intermediate portion 55 of the rod 52, which acts as a conductor, is smaller than the ends 39 and 31 which are in electrical communication with the dies 42 and 41 respectively. Therefore, in this specific form of apparatus the intermediate portion 55 is heated to a temperature which permits the metal to be' worked. Thereafter the platen 38 is moved downwardly to deform the intermediate portion 55 of the iridium rod 52.

The diameter of the enlarged shoulder section 32 is controlled by the length of the rod 52, the depth of the recesses 43 and 44 within the dies 41 and 42 and by the distance which the platen 38 is move-d toward the fixed bed 39.

Rather than electrical resistance type heating, the rod 52 may be heated by other means, for example, electrical or gas fired induction coils. The thermo-conductivity characteristics of the dies 42 and 41 may also be selectively chosen to differentially heat the rod 52 in a predetermined manner. For example, the dies 42 and 41 may be constructed of materials having different heat transfer properties, whereby a predetermined portion of the rod 52 is heated to a higher temperature because of the low thermo-conductivity of an adjacent die.

The method of the instant invention is not limited to producing tip electrodes having an outwardly extending intermediate portion, but also contemplates production of tips having outwardly extending end portions or end portions of various predetermined configurations.

The temperature to which the intermediate portion 55 of the rod 52 is heated may vary throughout a limited range, but if the temperature is too low, objectionable cracks or faults occur in the enlarged shoulder section 32 which is formed. However, in the preferred embodiment, surface cracks of a predetermined depth are beneficial. Such cracks insure the formation of a better bond between the shoulder section 32 and the core 25. These surface cracks should not, however, extend inwardly beyond the cylindrical surface projection defined by the extension of the firing end surface and the head end surface.

It has been found that the group of metals consisting of iridium, tungsten, molybdenum, ruthenium, and rhodium, reach a working range at a temperature generally below their recrystallization temperature.

It has also been determined that the temperature at which one of this group of metals recrystallizes varies with the period of time at which the metal remains at an elevated temperature. The recrystallization temperature increases as the time increment decreases. Therefore, it is possible to raise the temperature of, for example, iridium to approximately 2000 F. without substantial recrystallization if the increment of elevated temperature time is in the order of a fraction of a second, even though iridium, heated over a longer period of time, would normally recrystallize at 1800 F.

If severe metal recrystallization does occur, the completed article is too brittle to be used in a spark plug assembly. For example, referring to FIG. 1, the flange 26 normally is .030 inch thick and, therefore, offers little lateral support for the firing end 30 of the iridium tip 29. If recrystallization of the metal occurs in the firing end 30, there is a substantial probability that the firing end 30 will fail under normal usage and vibration.

In a tip electrode 29, constructed of iridium, when the center of the intermediate portion 55 is heated to approximately 2000 F. by the apparatus above-described, the portions of the rod 52 in the die recesses 43 and 44 remain well below the recrystallization temperature. It has further been found that if the intermediate portion 55 is held at this temperature for no more than approximately & of .a second, only a negligible amount of recrystallization occurs and this is confined to the enlarged shoulder section 32 and does not occur in either the firing end 30 or head end 31 which are subjected to the high est stress during usage.

Opposed surfaces of the dies 41 and 42 are constructed with various surface configurations to produce the shoulder section 32a, illustrated in FIG. 4 and the shoulder section 32b, illustrated in FIG. 5.

While the invention has been disclosed in conjunction 6 with a specific form and disposition of the parts, it should be expressly understood that numerous modifications and changes may be made without departing from the spirit and scope of the appended claims.

What I claim is:

1. In a spark plug having an insulating core with an axial bore therethrough, a center electrode mounted in such bore, an outer shell surrounding said insulating core, a ground electrode in electrical communication with said outer shell, said center electrode having a metallic sheath surrounding a metallic core at one end, the improvement comprising, a radially inwardly directed flange on a lower end of said metallic sheath, said flange defining a circumferential seat and a tip receiving opening, an upper surface of said flange having a predetermined seat configuration, an iridium tip coaxially aligned with said center electrode having a generally cylindrically shaped and longitudinally extending firing end, a head end longitudinally spaced from and axially aligned with said firing end, and an outwardly extending intermediate shoulder section positioned between and coaxially aligned with said firing end and said head end, said head end and said intermediate shoulder section positioned within said sheath forming a mechanical bond with said metallic core, said intermediate shoulder section of said iridium tip having a lower surface engaging and being seated upon said upper surface of said flange, and said firing end of said iridium tip positioned in a firing relationship with said ground electrode.

2. A tip electrode according to claim 1 wherein said outwardly extending intermediate shoulder seciion has an elliptical vertical cross section.

3. A spark plug comprising an insulating core with an axial bore therein, an outer shell surrounding said insulating core, a ground electrode in electrical communication with said outer shell, a center electrode mounted in said bore, said center electrode having a metal core, a radially inwardly directed flange at a lower end of said axial bore, said flange defining a circumferential seat and a tip-receiving opening, an upper surface of said flange having a predetermined seat configuration, an iridium tip coaxially aligned with said center electrode, said tip having a generally cylindrically shaped and longitudinally extending firing end, a head end longitudinally spaced from and axially aligned with said firing end, and an outwardly extending intermediate shoulder section positioned between and coaxially aligned with said firing end and said head end, said head end and said intermed'ate shoulder section positioned within said bore, said intermediate shoulder section being symmetrical with respect to a plane perpendicular to the axis of said iridium tip and extending centrally through said shoulder section, said intermediate shoulder section of said iridium tip having a lower surface engaging and being seated upon said upper surface of said flange, and said firing end of said iridium tip being positioned in a firing relationship with said ground electrode.

4. A method of making a spark plug having an insulating core with an axial bore therethrough, an iridium tipped center electrode mounted in such bore, an outer shell surrounding said insulating core, and a ground electrode in electrical communication with the outer shell, comprising the steps of placing an iridium rod between opposed pressure members, applying lateral support to upper and lower cylindrical surfaces of the iridium rod, heating an intermediate portion of the iridium rod to a working temperature by resistance heating, applying a compressive force to the opposed ends of the rod, moving the axially opposed ends of the iridium rod toward each other a predetermined distance thereby forming a shoulder section on the iridium rod, removing the shaped iridium tip from between the opposed pressure members, forming a radially inwardly directed flange near the lower end of the insulating core, aligning the iridium tip axial- 1y within the insulating core, seatingthe iridium tip on 2,837,679 6/1958 Schwartzwalder et al. the flange, and placing a metallic core in surrounding 313-136 X relationship with the upper end of the iridium tip there- 2,890,325 6/1959 Havlik 219-152 by forming a mechanical bond between the upper end of 2,891,187 6/1959 Bfetsch t al 313-136 the iridium tip and the metallic core. 5 2 12/1963 Candelise 313-136 X R f e Cit d b th E i JAMES W. LAWRENCE, Primary Examiner.

UNITED STATES PATENTS C. R. CAMPBELL, Assistant Examiner.

2,473,245 6/1949 Hanna 219-152 10 DAVID J. GALVIN, Examiner. 

1. IN A SPARK PLUG HAVING AN INSULATING CORE WITH AN AXIAL BORE THERETHROUGH, A CENTER ELECTRODE MOUNTED IN SUCH BORE, AN OUTER SHELL SURROUNDING SAID INSULATING CORE, A GROUND ELECTRODE IN ELECTRICAL COMMUNICATION WITH SAID OUTER SHELL, SAID CENTER ELECTRODE HAVING A METALLIC SHEATH SURROUNDING A METALLIC CORE AT ONE END, THE IMPROVEMENT COMPRISING, A RADIALLY INWARDLY DIRECTED FLANGE ON A LOWER END OF SAID METALLIC SHEATH, SAID FLANGE DEFINING A CIRCUMFERENTIAL SEAT AND A TIP RECEIVING OPENING, AN UPPER SURFACE OF SAID FLANGE HAVING A JPREDETERMINED SEAT CONFIGURATION, AN IRIDIUM TIP COAXIALLY ALIGNED WITH SAID CENTER ELECTRODE HAVING A GENERALLY CYLINDRICALLY SHAPED AND LONGITUDINALLY EXTENDING FIRING END, A HEAD END LONGITUDINALLY SPACED FROM AND AXIALLY ALIGNED WITH SAID FIRING END, AND AN OUTWARDLY EXTENDING INTERMEDIATE SHOULDER SECTION POSITIONED BETWEEN AND COAXIALLY ALIGNED WITH SAID FIRING END AND SAID HEAD END, SAID HEAD END AND SAID INTERMEDIATE SHOULDER SECTION POSITIONED WITHIN SAID SHEATH FORMING A MECHANICAL BOND WITH SAID METALLIC 